Transmissions by radio, light, and sonic energy have heretofore been largely approached from the point of view frequency content, or band of frequencies. Thus, and with respect to radio, coexistent different radio transmissions are permissible by means of assignment of different frequencies or frequency channels to different users, particularly those within the same geographic area. Essentially foreign to this concept is that of tolerating transmissions which are not frequency limited. While it would seem that the very notion of not limiting frequency response would create havoc with existing frequency denominated services, it has been previously suggested that such is not necessarily true and that, at least theoretically, it is possible to have overlapping use of the radio spectrum. One suggested mode is that provided wherein very short, on the order of one nanosecond or less, radio pulses are applied to a broadband antenna which ideally would respond by transmitting short burst signals, typically comprising three to four polarity lobes, which comprise, energywise, signal energy over essentially the upper entire band (above 100 megacycles) of the most frequently used radio frequency spectrum, that is, up to the midgigahertz region. A basic discussion of impulse effected radio transmission is contained in an article entitled "Time Domain Electromagnetics and Its Application," Proceedings of the IEEE, Vol. 66, No. 3, March 1978. This article particularly suggests the employment of such technology for baseband radar, and ranges from 5 to 5,000 feet are suggested. As noted, this article appeared in 1978, and now ten years later, it is submitted that little has been accomplished by way of achieving commercial application of this technology.
From both a theoretical and an experimental examination of the art, it has become clear to the applicant that the lack of success has largely been due to several factors. One is that the extremely wide band of frequencies to be transmitted poses very substantial requirements on an antenna. Antennas are generally designed for limited frequency bandwidths, and traditionally when one made any substantial change in frequency, it became necessary to choose a different antenna or an antenna of different dimensions. This is not to say that broadband antennas do not, in general, exist, but in general, applicant is unaware of any prior practical structures which, when excited by very short impulses, respond by the transmission of burst signals as described above, the ideal for this field of transmission. This view is based upon having tested many antennas and from discussions with contemporaries who are basically still struggling with the problem.
Two antenna types have received attention as being reasonably good broadband radiators, or receivers--the bicone antenna and various forms of horn antennas, particularly wherein the antenna becomes an extension of a feed transmission line. The applicant has tested published versions of both and has found that they simply fail to meet the obvious goal of transmitting sufficiently short bursts. Recently, applicant has learned of an improved horn-type antenna with improved response. However, it is understood to be three-dimensionally large and thus appears impractical for most common uses.
A second problem which has plagued advocates of the employment of impulse or time domain technology for radio is that of effectively receiving and detecting the presence of the signal bursts, particularly in the presence of high levels of existing ambient radiation, present nearly everywhere. If one considers the problem simply in terms of competition with the ambient signals, it might appear insurmountable, and perhaps this is an explanation for the lack of progress in receiver technology in this field. The state of the art prior to applicant's entrance generally involved the employment of brute force detection, that of threshold or time threshold gate detection. Threshold detection simply enables passage of signals higher than a selected threshold level. The problem with this approach is obvious in that if one transmits impulse generated signals which are of sufficient amplitude to rise above ambient signal levels, the existing radio services producing the latter may be unacceptably interferred with. For some reason, perhaps because of bias produced by the wide spectrum of signal involved, e.g., from 50 MHz on the order of 5 GHz, the possibility of coherent detection has been thought impossible.
With respect to transmissions via light and sonic energy, conventional techniques similarly call for relatively narrow frequency band transmissions which require quite high spectral density of frequency energy, and this in turn has been, in certain applications, a disadvantage that can be detected by unintended receivers.
Accordingly, it is the object of this invention to provide an impulse or time domain (or baseband) transmission system which attacks all of the above problems and to provide a complete impulse time domain transmission system which, in the applicant's view, eliminates the known practical barriers to its employment, and, importantly, its employment for electromagnetic and sonic modes of radio transmission, including communications, telemetry, navigation, radar, and sonar.